1. Field of the Invention
This invention relates to a method for modulating or controlling the position of valves used in hydronic heating systems, which systems transfer a heat medium such as water to heat a radiation device to provide radiant heat. Conventionally, such radiant heating systems may be used in the home or commercially, and can be used to heat large areas such as floors or ceilings.
2. Description of Related Art
Conventional hydronic heating systems generally have a primary system in which a boiler is engaged to heat the water and a secondary system into which the water from the primary system flows under certain controlled conditions. Although the system and control method are chiefly herein described with regard to a heating system, they apply equally to a cooling system in which fluid which is cooled is carried to the system in which a cooling effect is to be achieved.
Transfer of a heated or cooled fluid medium between primary and secondary systems is accomplished by means of multi-port control valves to be described below. These valves are generally motor controlled, expensive, and are sometimes difficult to operate when attempting to achieve certain desired heating or cooling effects.
The following is a description of specific prior art hydronic heating systems generally employed. In this description, reference is made to FIGS. 1-5.
A conventional hydronic heating system is illustrated in FIG. 1. The system consists of a boiler 1 used to heat a transfer medium (e.g., water), and a pump 2 to move the heated transfer medium from boiler 1 to a transfer device 3 (e.g., radiation) to transfer the heat from the heated medium to the space to be heated. The heated transfer medium is returned to boiler 1 at a lower temperature than it left the boiler after transferring some of its heat to transfer device 3.
In a basic hydronic heating system, boiler 1 heats water to the required temperature needed to be delivered to transfer device 3 used to heat the space. This transfer device typically would be a cast iron vessel or a copper tube with fins that is heated by the passage of heated water through it. In certain applications it is necessary to have the water turned on and off to different areas or zones to be heated. To accomplish this, valves are used in the branch flow to each zone.
FIG. 2 shows the piping arrangement of a multiple zone system. Pump 5 pumps water from boiler 4 to different heating zones 7a, 7b, and 7c. Each piping branch is provided with a zone valve, 6a, 6b, and 6c, respectively. Depending on the state of a zone valve 6a, 6b, or 6c, i.e., whether it is open or closed, heated boiler water is either sent to the corresponding heating zone 7a, 7b, or 7c or blocked. When a zone valve is open, heated water is delivered to a corresponding radiation device, and the zone is heated. When a zone valve is closed, heated water is not delivered to its corresponding radiation device, and the zone is not heated. With this type of control, each zone is controlled solely in an on-off fashion. These valves are generally of the inexpensive solid element type.
FIG. 3 shows an internal view of such a solid element valve. The typical valve possesses a piston 10 movable within valve 11 between open and closed positions. An electric heater 8 is in thermal communication with a cylinder 9. Cylinder 9 is filled with a substance such as wax which expands when heated. When powered, electric heater 8 heats cylinder 9 which expands and displaces piston 10 thereby opening or closing valve 11.
In certain cases, particularly with radiant heat devices, it is desirable to reduce the flow of water to a zone in order to lessen the amount of heat delivered. Since the conventional zone valve does not lend itself easily to a continuously modulating or variable mode of operation, a three-way or four-way valve could be installed for each zone.
FIG. 4 illustrates the piping arrangement of a three-way mixing valve for a single zone; like structures in other zones of a multiple zone system are not illustrated. Depending on the position of the control port in three-way valve 105, all, some, or none of the boiler water flows to radiation system 107. When the control port in three-way valve 105 is positioned so that all of the boiler water flows to radiation system 107 (the 100% position), boiler port 105a is connected to output port 105b, radiation system 107 receives water at the boiler temperature, there is no flow in return port 105c, and all of the flow from radiation system 107 is returned to boiler 104. Thus, when valve 105 is in the 100% position, the system functions no differently than the system shown in FIG. 1. When valve 105 is in a 0% boiler water position, return port 105c is connected to the output port 105b, the radiation system 107 receives water at the returned water temperature of the radiation system, and there is no flow in boiler port 105a. In the 0% boiler position, no heated water from boiler 104 flows to radiation system 107, and the radiation system remains at the ambient temperature.
When the port of the valve is in some mid-way position, some percentage of the flow is through boiler port 105a, and the remaining percentage of the flow is through return port 105c. By blending or mixing the water leaving the boiler with water that has lost some of its heat in the radiation system, water having a temperature lower than that of the boiler water may be supplied to the radiation system. By varying the boiler port position between 0 and 100%, the temperature supplied to the radiation system may be varied between the ambient temperature of the radiation system and the boiler water temperature. In this configuration, the flow through the radiation system remains constant but the flow through the boiler varies with the position of the valve. Varying the flow through the boiler may be problematic, as some boilers are extremely flow-sensitive, and can only operate within a narrow range of flow rates. If varying the flow presents a problem, a four-way valve might be employed to maintain a constant flow through the boiler and radiation system in all valve positions.
The four-way valve is piped into a system as illustrated in FIG. 5. As before, FIG. 5 illustrates the piping arrangement of a four-way valve for a single zone; like structures in other zones of a multiple zone system are not illustrated. When valve 209 is set in a valve position of 100% boiler water, all boiler water flows into boiler port 209a out to radiation system 210 through system supply port 209c, and the water returns from radiation system 210 into system return port 209d and back to boiler 204 via return port 209b. When valve 209 is set in a 0% boiler water valve position, boiler water enters boiler port 209a and returns to boiler 204 through boiler return port 209b, while water in the radiation side of the valve moves out of the system supply port 209c and returns to the valve through system return port 209d. In positions between 0 and 100%, a regulated amount of boiler water mixes with the water moving through the radiation, allowing control of the water temperature going to the radiation between the ambient temperature of the radiation and the boiler water temperature.
The system illustrated in FIG. 5 has what are referred to as primary and secondary loops, with high temperature water flowing through the primary loop (the boiler loop) and lower temperature water flowing through the secondary loop (the radiation). Both systems illustrated in FIGS. 4 and 5 utilize mixing valves, which are expensive and can be complicated to operate.
One known valve control method involves a simple solid element valve having a heatable expandable element as shown in FIG. 3, and consists of varying the duty cycle of the control signal (the "open" command) to the valve. By varying the duty cycle of the control signal to the heater of the valve, the valve can open and close in a variable manner in an attempt to control fluid temperature levels. Such a method is mentioned in U.S. Pat. No. 4,666,081 to Cook et al., the teachings of which are incorporated by reference herein.
The Cook system of simply varying the duty cycle of the control signal is flawed for the following reasons. The present inventors have found that such a system does not work satisfactorily, owing to the long delay from when power is first supplied to the heater on the wax expandable element to the time when the valve actually begins to open. This lag time or delay causes the pulse width modulation control logic to continually increase the "on time" of the output signal with respect to the "off time" during the time it takes the heater to get hot enough to move the valve. Once the valve finally begins to open, the duty cycle of the pulse width modulation output ends up being too great and causes the valve to open too far for the required flow; the desired temperature target is overshot. As a result, the system is unstable and oscillates at a low frequency. The present inventors discovered that, in a closed loop control system, the time to heat the expandable element to the point at which it begins to actively control the valve (i.e., to the point where the valve is accurately responsive to the pulsed control signal) needs to be controlled. Cook et al. do not address this problem with using a valve having a heat-expandable element, nor do Cook et al. address the problem with regard to any other type of valve having a electric actuator in which an inherent lag time in powering up is present.